JP7410790B2 - Open magnetic resonance imaging device - Google Patents

Description

The present invention relates to an open magnetic resonance imaging apparatus.

A magnetic resonance imaging device (hereinafter referred to as an MRI device) irradiates a subject placed in a uniform static magnetic field with high-frequency pulses to obtain cross-sectional images showing the physical and chemical properties of the subject through nuclear magnetic resonance phenomena. It is a device. It is mainly used for medical purposes.
An MRI apparatus mainly includes 1) a static magnetic field magnet, 2) a gradient magnetic field coil, 3) an RF coil, 4) a receiving coil, and 5) a computer system.
The static magnetic field magnet generates a uniform static magnetic field in the imaging space into which the subject is inserted.
The gradient magnetic field coil generates a pulsed magnetic field with a spatially gradient strength in order to add position information to imaging data.
The RF coil irradiates the subject with high frequency pulses, and the receiving coil receives magnetic resonance signals from the subject.
A computer system processes the received signals to represent an image.

The gradient magnetic field coil generates a gradient magnetic field within the imaging space, and also generates a fluctuating magnetic field (leakage field) outside the imaging space that is unnecessary for imaging.
The leakage magnetic field generates eddy currents in metal structures that constitute the MRI apparatus, such as static magnetic field magnets.
Eddy currents generate a fluctuating magnetic field within the imaging space, which affects the distribution of the static magnetic field and gradient magnetic field, and becomes a factor that degrades image quality.
Further, in open-type MRI apparatuses, iron magnetic poles are often used for static magnetic field magnets. The fluctuating magnetic field caused by the leakage magnetic field also affects the magnetization of the iron magnetic pole, which has hysteresis characteristics. Furthermore, changes in magnetization affect the distribution of the static magnetic field and the gradient magnetic field, and become a factor in degrading image quality.

For this reason, recent MRI apparatuses use a method of suppressing magnetic field leakage to metal structures by using gradient magnetic field coils that are self-shielding, that is, equipped with active shields.
A problem with gradient coils with active shields is that the static field magnet must be moved away from the gradient coil and the imaging region by the thickness of the shield. Then, the magnetic energy of the static field magnet increases, potentially making the MRI apparatus expensive.
Further, such a gradient magnetic field coil has a large inductance and requires a driving power source with a large output, so the MRI apparatus may become expensive due to this point as well.
In connection with the above-mentioned fields, there are Patent Document 1 and Patent Document 2 regarding open type MRI apparatuses with medium and low magnetic fields (less than 0.5 T).

The [Summary] of Patent Document 1 states, ``[Objective] To reduce the generation of eddy currents and bring the gradient magnetic field to a predetermined strength in a short time without reducing the magnetic field uniformity in the air gap of a magnetic field generator for MRI. Providing a magnetic pole piece with a configuration that can rise and reduce the residual magnetism phenomenon to obtain a highly sensitive and clear image. [Configuration] Multiple blocks laminated using non-oriented silicon steel plates. A laminated silicon steel plate layer 11 formed of a magnetic pole piece member, a soft iron magnetic ring 12 with a rectangular cross section surrounding the laminated silicon steel plate 11, and soft ferrite powder compressed into a rectangular plate shape. The magnetic field in the air gap is made uniform by the magnetic pole piece 10, which is made up of a soft ferrite layer 13 formed by combining a large number of molded block-shaped magnetic pole piece members with adhesive into a disk shape and laid on the upper surface of the laminated silicon steel plate 11. It is easy to achieve, and even when GC pulses are applied to the gradient magnetic field coils, the eddy currents generated in the magnetic poles are reduced, and the residual magnetism phenomenon can be reduced.'', and the technology of the magnetic field generator for MRI is disclosed. has been done. In addition, a countermeasure method is disclosed in which a magnetic pole piece (for example, soft ferrite or silicon steel plate) is provided between the gradient magnetic field coil and the metal structure to serve as a path for magnetic flux and to suppress the generation of eddy current.

The [Summary] of Patent Document 2 states, ``[Problem] It is possible to suppress the generation of eddy current magnetic fields due to leakage magnetic fields and fluctuating magnetic fields due to magnetization changes in iron magnetic poles when gradient magnetic fields are generated, and at the same time widen the imaging space. To provide an MRI apparatus equipped with a means for obtaining good images while improving the comfort of an examiner. [Solution Means] In an MRI apparatus having a static magnetic field magnet having magnetic poles and a gradient magnetic field coil, the imaging space is The opposing magnetic poles are composed of a substantially disc-shaped iron magnetic pole, a substantially annular iron magnetic pole, and a substantially tile-shaped silicon steel plate magnetic pole piece, and the tile-shaped silicon steel plate is attached to the surface of the disc-shaped iron magnetic pole. The disc-shaped iron magnetic pole is divided in the circumferential direction and insulated from each other by an insulator or an air gap.'', and the technology of the magnetic resonance imaging apparatus is disclosed. Furthermore, a method is disclosed in which a magnetic pole is divided in the circumferential direction to attenuate the generated eddy current.

Japanese Patent Application Publication No. 05-182821 Japanese Patent Application Publication No. 2016-96829

However, with the countermeasure method disclosed in Patent Document 1, it is difficult to completely shield the leakage magnetic field due to its structure, and there is a problem (problem) in which eddy currents occur in the disc-shaped magnetic poles that constitute the static magnetic field magnet. )was there.
In addition, the method disclosed in Patent Document 2 that divides magnetic poles in the circumferential direction and attenuates the generated eddy current requires high processing and processing costs in order to divide the magnetic poles while ensuring uniformity of the static magnetic field. There was a problem that assembly precision was required and the manufacturing cost of static magnetic field magnets increased. In addition, when a magnetic material magnet is divided, its effective saturation magnetization decreases, so higher magnetic energy is required to achieve the same static magnetic field strength as an undivided static field magnet, making MRI equipment expensive. There was an issue of becoming.

An object of the present invention is to improve the image quality of an MRI image (Magnetic Resonance Imaging) of an open-type magnetic resonance imaging apparatus and to provide a low-cost open-type MRI apparatus.

In order to solve the above problems, the present invention was configured as follows.
That is, the open magnetic resonance imaging apparatus of the present invention includes: a pair of static magnetic field magnets arranged opposite to each other with the imaging space at the center; a pair of gradient magnetic field coils arranged opposite to each other around the imaging space; The static magnetic field magnet includes a disk-shaped magnetic pole that generates a static magnetic field in the Z-axis direction, which is the direction in which the pair of static magnetic field magnets face each other, and a static magnetic field that generates a static magnetic field in the XY plane perpendicular to the Z-axis direction. an annular magnetic material magnetic pole; the gradient magnetic field coil shields a Z coil that provides a magnetic field tilted in the Z-axis direction in an imaging space; and a magnetic flux generated from the Z coil to the disk-shaped magnetic material magnetic pole. It is characterized by comprising a magnetic block and a correction coil that shields the magnetic flux generated from the Z coil from the annular magnetic pole.

Further, other means will be explained in the detailed description.

According to the open magnetic resonance imaging apparatus of the present invention, it is possible to suppress eddy currents caused by leakage magnetic fields of gradient magnetic field coils and improve the image quality of MRI images without dividing the magnetic poles constituting the static magnetic field magnet. .
Further, since the magnetic field strength can be achieved with lower magnetic energy and high processing and assembly precision is not required, a low-cost open-type MRI apparatus can be provided.

1 is a diagram showing a cross-sectional configuration example of an open magnetic resonance imaging apparatus according to a first embodiment of the present invention; FIG. 1 is a diagram showing a schematic configuration example of a static magnetic field magnet of the open magnetic resonance imaging apparatus according to a first embodiment of the present invention, and a configuration example of a gradient magnetic field coil and an RF coil arranged in the vicinity of the static magnetic field magnet. FIG. 2 is a diagram illustrating a schematic configuration example of a cross section of an XZ plane of a gradient magnetic field coil of an open magnetic resonance imaging apparatus according to a first embodiment of the present invention, viewed from the Y direction. 3A is a diagram showing a bird's-eye view of an example of the configuration of the gradient magnetic field coil of FIG. 3A. FIG. FIG. 2 is a diagram illustrating a schematic configuration example of a cross section of the XY plane of the X coil in the gradient magnetic field coil of the open magnetic resonance imaging apparatus according to the first embodiment of the present invention, viewed from the Z direction. 4A is a diagram showing a cross section of the XZ plane from the Y direction, showing an example of the flow of magnetic flux that occurs when a pulse current is passed through the X coil of FIG. 4A. FIG. FIG. 3 is a diagram illustrating a schematic configuration example on the XY plane of a Z coil and a correction coil in a gradient magnetic field coil of the open magnetic resonance imaging apparatus according to the first embodiment of the present invention. 5A is a diagram showing an example of the flow of magnetic flux that occurs when a pulse current is passed through the Z coil and the correction coil shown in FIG. 5A. FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, modes for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the drawings as appropriate.

≪First embodiment≫
An open magnetic resonance imaging apparatus (open MRI apparatus) according to a first embodiment of the present invention will be described with reference to the drawings.

<Cross-sectional configuration of open magnetic resonance imaging device>
FIG. 1 is a diagram showing a cross-sectional configuration example of an open type magnetic resonance imaging apparatus (hereinafter, appropriately referred to as an "open type MRI apparatus") according to a first embodiment of the present invention.
In FIG. 1, an open-type MRI apparatus 100 includes a pair of upper and lower static magnetic field magnets 101, a support section 106 that supports and holds the pair of upper and lower static magnetic field magnets 101, and a movable bed 105.
In addition, the subject (subject, subject) 104 in the open MRI apparatus 100 rides on the movable bed 105, and as the movable bed 105 moves, the space between the pair of upper and lower static magnetic field magnets 101 is Placed.

The open-type MRI apparatus 100 drives a pair of upper and lower static magnetic field magnets 101 to generate a static image in the direction of the Z-axis 103z in a substantially spherical imaging space (imaging region) 102 located between the upper and lower pair of static magnetic field magnets 101. Generates a magnetic field.
Note that the Z-axis 103z passes through the center of the imaging space 102 as its origin. Two axes perpendicular to the Z-axis 103z at the origin are defined as an X-axis 103x and a Y-axis 103y, respectively.
Further, the imaging space 102 has static magnetic field magnets 101 above and below (Z-axis direction), but is open in the horizontal direction (Y-axis direction).

The subject 104 is transported to the imaging space 102 by the movable bed 105, as described above. Then, the open MRI apparatus 100 acquires an MRI image of the subject 104.
Since the imaging space 102 of the open MRI apparatus 100 is not surrounded by structures, the subject 104 can feel a sense of freedom compared to horizontal magnetic field MRI.
Furthermore, by moving the movable bed 105, the affected area can always be imaged at the center of the imaging space 102. Another feature is that an operator of the open MRI apparatus or the like can assist in imaging the subject 104 from outside the open MRI apparatus 100 while the MRI image is being taken.

<Schematic configuration of static magnetic field magnet, gradient magnetic field coil, and RF coil>
FIG. 2 shows a schematic configuration example of the static magnetic field magnet 101 of the open magnetic resonance imaging apparatus according to the first embodiment of the present invention, and a gradient magnetic field coil 205 and an RF coil 206 arranged near the static magnetic field magnet 101. It is a figure showing an example of composition.
In FIG. 2, a static magnetic field magnet 101, a gradient magnetic field coil 205, and an RF coil 206 are illustrated.
Also, an X-axis 103x, a Y-axis 103y, a Z-axis 103z, and an imaging space 102 are shown.

However, for the sake of simplicity, only the upper side (positive side of the Z axis) and the right side (positive side of the X axis) of the pair of upper and lower static magnetic field magnets 101 are described.
The configuration inside the region indicated by the broken line indicates the static magnetic field magnet 101. Note that the gradient magnetic field coil 205 and the RF coil 206 will be described later.
In addition, although not shown, the open MRI apparatus (100) includes a power supply device for driving the gradient magnetic field coil 205 and the RF coil 206, a power supply device for controlling the power supply, and an RF coil 206 (for signal generation). It also includes a computer system that images the signals obtained by the receiver coil (which also serves as a receiving coil).

《Static magnetic field magnet》
The static magnetic field magnet 101 generates a uniform static magnetic field in the imaging space 102 into which the subject (104: FIG. 1) is inserted.
As shown in FIG. 2, the static magnetic field magnet 101 includes a disc-shaped magnetic pole 201, an annular magnetic pole 202, a shield coil 203, and a main coil 204. It is also provided with a return yoke (not shown) (made of electromagnetic steel plate, whose role is to prevent lines of magnetic force from leaking to the outside).
The static magnetic field magnet 101 has a substantially axially symmetrical structure about the Z-axis 103z.

The disk-shaped magnetic material magnetic pole 201 is made of a magnetic material such as iron, and has a disk shape centered on the Z-axis 103z. However, in order to form a desired magnetic field, it has a predetermined uneven shape along the X-axis direction and in the Z-axis direction.
The annular magnetic material magnetic pole 202 is made of a magnetic material such as iron, and has an annular shape centered on the Z-axis 103z.
Note that the disc-shaped magnetic material magnetic pole 201 is configured in a disc shape, and the annular magnetic material magnetic pole 202 is configured in an annular shape, and both have no slit in the magnetic material and are undivided. Therefore, compared to the case of dividing, manufacturing is easier and manufacturing cost is lower.
Furthermore, since the magnetic material is not divided, the magnetic energy efficiency is higher than that in the case where the magnetic material is divided.

Further, in FIG. 2, a disc-shaped magnetic pole 201 and an annular magnetic pole 202 form a magnetic circuit.
The main coil 204 is composed of a substantially annular coil centered on the Z-axis.
By passing current through each of the upper and lower pair of main coils 204, a static magnetic field as the upper and lower pair of static magnetic field magnets 101 is formed including the imaging space 102 via the magnetic circuit.

The magnetic field formed by said magnetic circuit may leak and be generated outside the open MRI apparatus (100: FIG. 1). In order to reduce the magnetic field outside the open type MRI apparatus 100, a substantially annular shield coil 203 is provided that generates a magnetic field in the opposite direction to the imaging space.
The shield coil 203 generates a magnetic field in the opposite direction to the magnetic field formed in the imaging space by the magnetic circuit, thereby reducing the magnetic field outside the open MRI apparatus 100.
Furthermore, as described above, the return yoke (not shown) also serves to reduce the magnetic field outside the open MRI apparatus 100.

As described above, the static magnetic field is generated by the approximately annular main coil 204 made of a conductive material (for example, a superconducting material).
Note that when superconducting coils are used for the main coil 204 and the shield coil 203, the coils used for the main coil 204 and the shield coil 203 have a heat insulating structure such as a vacuum container, a radiation shield, a liquid helium container, etc. from the outside. It is housed in a container (not shown) and maintained at an extremely low temperature using liquid helium, a refrigerator (not shown), and the like.

In FIG. 2, on the imaging space 102 side of the static magnetic field magnet 101, a pulsed gradient magnetic field is generated in the same direction as the Z-axis 103z, which is the direction of the static magnetic field, with an intensity proportional to the distance from the center of the imaging space 102. A gradient magnetic field coil 205 is installed.
Further on the imaging space 102 side of the gradient magnetic field coil 205, a substantially disc-shaped RF coil 206 that generates a high-frequency electromagnetic pulse (RF pulse) is installed.
Next, the gradient magnetic field coil 205 and the RF coil 206 will be explained. However, since the features of the present invention mainly lie in the static magnetic field magnet 101 and the gradient magnetic field coil 205, the outline of the RF coil 206 will be explained first, and then the configuration of the gradient magnetic field coil 205 will be explained in detail.

《RF coil》
The RF coil 206 shown in FIG. 2 generates an electromagnetic wave (vibration irradiate (magnetic field).
By irradiating this electromagnetic wave, an RF magnetic field (RF pulse) is applied in a direction perpendicular to the static magnetic field. The applied RF pulse is an oscillating magnetic field, and a component of this oscillating magnetic field generates an RF magnetic field rotating around the Z axis.
This rotating RF magnetic field reacts with and resonates with the spins of various atomic nuclei forming the subject 104, and the state of the macroscopic nuclear spins as a whole is detected as a signal.

The RF coil 206 has the role of applying an RF magnetic field (RF pulse, oscillating magnetic field) in a direction perpendicular to the static magnetic field, and also reacts with the spins of various atomic nuclei forming the subject (subject) 104 to create a state of resonance. It may also serve as a receiving coil for receiving signals. Alternatively, a receiving coil (not shown) is provided in FIG.
Note that the signal related to magnetic resonance from the subject 104 received by the receiving coil is sent to a computer (not shown). The computer system processes the received signals and represents them as images (MRI images).

《Gradient magnetic field coil》
The gradient magnetic field coil 205 has a function of generating a pulsed magnetic field with a spatially gradient strength in order to add position information to imaging data.
FIG. 3A is a diagram showing a schematic configuration example of a cross section of the XZ plane of the gradient magnetic field coil of the open magnetic resonance imaging apparatus (open type MRI apparatus) according to the first embodiment of the present invention, viewed from the Y direction. be.
FIG. 3B is a diagram showing a bird's-eye view of a configuration example of the gradient magnetic field coil 205 in FIG. 3A.

In FIG. 3A, the gradient magnetic field coil 205 includes a gradient magnetic field generating coil 301, a magnetic block 302, and a correction coil 303.
Further, in FIG. 3A, an X-axis 103x, a Y-axis 103y, a Z-axis 103z, and an imaging space 102 are shown.
The gradient magnetic field coil 205 has a substantially axially symmetrical structure about the Z-axis 103z.

[Gradient magnetic field generation coil 301]
In FIG. 3A, the gradient magnetic field coil 205 includes three sets of gradient magnetic field generating coils 301 (X coil 301x, Y coil 301y, Z coil 301z) in the vertical direction.
Three sets of gradient magnetic field generating coils 301 (X coil 301x, Y coil 301y, Z coil 301z) generate magnetic fields in three directions perpendicular to the imaging space 102 (the direction of the X axis 103x, the direction of the Y axis 103y, and the direction of the Z axis 103z). direction) to generate gradient magnetic fields independently.

As described above, the gradient magnetic field coil 205 generates a pulsed magnetic field with a spatially gradient intensity in order to add position information to the imaging data.
However, while the gradient magnetic field coil 205 generates a gradient magnetic field within the imaging space, it also generates a fluctuating magnetic field (leakage field) outside the imaging space that is unnecessary for imaging.
The leakage magnetic field generates eddy currents in metal structures that constitute the open type MRI apparatus 100, such as static magnetic field magnets.
Eddy currents generate a fluctuating magnetic field within the imaging space, which affects the distribution of the static magnetic field and the gradient magnetic field, and becomes a factor in degrading the image quality of MRI images.
Further, details of the X coil 301x (and Y coil 301y) will be described later with reference to FIGS. 4A and 4B.
Further, details of the Z coil 301z will be described later with reference to FIGS. 5A and 5B.

[Magnetic block 302]
In FIG. 3A, the magnetic block 302 is provided on the static magnetic field magnet 101 side of the gradient magnetic field generation coil 301.
The magnetic block 302 includes a plurality of sheet-like magnetic bodies 302a (FIG. 3B) that are magnetic bodies with low magnetic resistance.
As shown in FIG. 3B, a plurality of sheet-like magnetic bodies 302a are stacked in the Z-axis 103z direction. Further, the plurality of sheet-like magnetic bodies 302a are configured by arranging magnetic bodies with low magnetic resistance in the shape of tiles in the X-axis 103x direction and the Y-axis 103y direction.

With the above configuration, the magnetic block 302 serves to shield the leakage magnetic field generated by the gradient magnetic field generating coil 301 on the static magnetic field magnet 101 side.
Note that the material of the sheet-like magnetic body 302a is arbitrary depending on the required performance of the open-type MRI apparatus 100, but a silicon steel plate with high magnetic permeability and high electrical resistance is preferable.
Note that high magnetic permeability corresponds to low magnetic resistance.
Furthermore, the fact that the electrical resistance is large and the sheet-like magnetic bodies 302a are arranged in a tile shape has the effect of suppressing the generation of eddy current in the magnetic body block 302.

The thickness of the magnetic block 302 is arbitrary depending on the required performance of the open-type MRI apparatus 100, but when a silicon steel plate is used and shielding of the magnetic flux reaching the disc-shaped magnetic pole 201 is taken into account. It is desirable that the thickness be at least 20 mm or more.
By setting the thickness of the magnetic block 302 to 20 mm or more, the inductance of the magnetic block 302 and the gradient magnetic field coil 205 is reduced.
Note that when the gradient magnetic field coil 205 has a low inductance, the burden on the power source is reduced, which also leads to a reduction in power source cost.
Furthermore, if the magnetic block 302 is not provided, the magnetic flux (magnetic field) generated from the gradient magnetic field generating coil 301 (Z coil 301Z) will reach the disc-shaped magnetic pole 201, as shown in FIGS. 4B and 5B, which will be described later. do. In order to prevent this, it is necessary to increase the distance between the gradient magnetic field generating coil 301 and the disc-shaped magnetic pole 201. Comparing this spacing with the space in the Z-axis direction due to the provision of the magnetic block 302, the provision of the magnetic block 302 makes a greater contribution to suppressing the thickness of the static magnetic field magnet 101.

[Correction coil 303]
In FIG. 3A, the correction coil 303 is arranged in an annular shape at the outer periphery of the Z coil 301z and at the same height.
The correction coil 303 arranged in this manner has a function of guiding the magnetic flux generated from the Z coil 301z and flowing through the magnetic block 302 in a direction opposite to the Z axis 103z. That is, the correction coil 303 cancels the magnetic flux generated from the Z coil 301z and flowing through the magnetic block 302.
The installation position of the correction coil 303 is arbitrary depending on the required performance of the open type MRI apparatus 100, but when considering shielding of the magnetic flux reaching the annular magnetic pole 202, as described above, the installation position of the correction coil 303 is It is desirable to install it outside the coil 301z and at the same height.

[X coil 301x]
The X coils 301x of the three sets of gradient magnetic field generating coils 301 described above will be described in detail with reference to FIGS. 4A and 4B. Note that in FIG. 3A, the configuration of the X coil 301x when viewed from the Y direction in a cross section of the XZ plane is explained. Next, in FIG. 4A, the configuration of the X coil 301x when viewed from the Z direction in the cross section of the .
FIG. 4A shows a cross section of the X coil 301x in the gradient magnetic field coil 205 of the open magnetic resonance imaging apparatus (open MRI apparatus) 100 according to the first embodiment of the present invention on the XY plane when viewed from the Z direction. FIG. 4 is a diagram illustrating a schematic configuration example of an X coil 401. FIG.
FIG. 4B is a diagram showing a cross section of the XZ plane from the Y direction, showing an example of the flow of magnetic flux 402 generated by the X coil 301x when a pulse current is passed through the X coil 301x.
In FIGS. 4A and 4B, the X coil 301x generates a gradient magnetic field in the same direction as the Z axis 103z, which is the direction of the static magnetic field, with respect to the X direction in the imaging space 102 (FIG. 4B).

In FIG. 4A, the X coil 301x includes an X coil 301xa and an X coil 301xb.
The X coil 301xa is located on the left side of the Z-axis 103Z when viewed from the paper. The coil of the X coil 301xa is spirally wound. In the X coil 301xa, the current flows in a clockwise direction as viewed from the paper, as shown by a current direction 1401a.
The X coil 301xb is located on the right side of the Z-axis 103Z when viewed from the paper. The coil of the X coil 301xb is spirally wound. In the X coil 301xb, the current flows in a counterclockwise direction as viewed from the paper, as shown by a current direction 1401b.
Therefore, as described later in FIG. 4B, the direction of the generated magnetic field (magnetic flux) is opposite between the X coil 301xa and the X coil 301xb.

In FIG. 4B, the disc-shaped magnetic pole 201, the magnetic block 302, and the X coil 301x are shown as a configuration of the X coil 401 on the XY plane when the cross section of the XY plane is viewed from the Z direction.
In addition, in FIG. 4B, although the X coil 301x of the gradient magnetic field generation coil 301 is shown, the Y coil 301y and the Z coil 301z are not shown.

The X coil 301xb on the right side of the X coil 301x shown in FIG. 4A mainly generates magnetic flux in the direction of the magnetic flux 1430b in FIG. 4B.
The X coil 301xa on the left side of the X coil 301x shown in FIG. 4A mainly generates magnetic flux in the direction of the magnetic flux 1430a in FIG. 4B.
As described above with reference to FIG. 4A, the current direction of the right side X coil 301xb and the left side X coil 301xa is opposite, so in FIG. 4B, the magnetic flux directions of the magnetic flux 1430b and the magnetic flux 1430a are opposite. be.

Therefore, in FIG. 4B, the magnetic flux 1430b of the right X coil 301xb is directed upward (in the positive direction of the Z axis) when viewed from the paper. Furthermore, the magnetic flux 1430a of the left X coil 301xa is directed downward (in the negative direction of the Z axis) when viewed from the paper.
As described in FIG. 3B, the magnetic block 302 has high magnetic permeability and high electrical resistance, that is, low magnetic resistance.
Therefore, the magnetic flux 1430b generated by the X coil 301xb on the right and directed upward in the paper (positive direction of the Z axis) passes through the magnetic block 302 with low magnetic resistance as the magnetic flux 1430, and as seen in the paper towards the left (magnetic flux 1430).
Then, it becomes a magnetic flux 1430a generated in the left X coil 301xa, and heads downward (in the negative direction of the Z axis) as viewed from the paper.
In this way, a flow of magnetic flux (1430b, 1430, 1430a) is formed.

In FIG. 4B, if there is no magnetic block 302 with low magnetic resistance, which is a shield-like magnetic substance, instead of the magnetic flux 1430, the magnetic flux 1420 penetrates through the magnetic block 302 and becomes a disk-shaped magnetic flux. It reaches the body magnetic pole 201. Such a leakage magnetic field becomes a cause of generation of eddy current in the disc-shaped magnetic pole 201.

As described above, with the structure shown in FIGS. 4A and 4B, the magnetic flux of the Generation of current can be suppressed. That is, the eddy current caused by the leakage magnetic field of the gradient magnetic field coil (X coil 301x) is suppressed.

[Y coil 301y]
Returning to FIG. 3A, the Y coil 301y of the three sets of gradient magnetic field generating coils 301 described above will be described.
Since the flow of magnetic flux in the Y coil 301y is symmetrical with respect to the flow of magnetic flux in the X coil 301x described above with respect to the Y axis and the X axis, a redundant explanation will be omitted.
However, while the X coil 301x generates a gradient magnetic field in the X-axis direction, the Y coil 301y generates a gradient magnetic field in the Y-axis direction.
Furthermore, since the magnetic block 302 has a disk shape as shown in FIGS. 3A and 3B, it has the same effect on the Y coil 301y as it does on the X coil 301x.

[Z coil 301z]
The detailed configuration of the Z coil 301z of the three sets of gradient magnetic field generating coils 301 shown in FIG. 3A will be described in detail with reference to FIGS. 5A and 5B.
FIG. 5A schematically shows the Z coil 501 on the XY plane of the Z coil 301z in the gradient magnetic field coil 205 of the open magnetic resonance imaging apparatus (open MRI apparatus) 100 according to the first embodiment of the present invention, and the correction coil 303. FIG. 3 is a diagram showing an example of the configuration and the direction of current flowing in each.
FIG. 5B is a diagram showing an example of the flow of the magnetic flux 502 generated by the Z coil 301z shown in FIG. 5A and the correction coil 303 when a pulse current is passed through the Z coil 301z.
The Z coil 301z generates a gradient magnetic field in the same direction as the direction of the static magnetic field (direction of the Z axis 103z) with respect to the Z direction on the imaging space 102 side.

In FIG. 5A, a current direction 1501 of a current flowing through a Z coil 301z formed by winding a plurality of times in an annular shape and a current direction 1503 of a current flowing through a correction coil 303 formed by winding a plurality of times in an annular shape are as follows. It is in the opposite direction.
When a current is passed through the Z coil 301z in FIG. 5A in the direction of the current direction 1501 with the center of the Z axis, as shown in FIG. 5B, magnetic flux (1530b, 1531b) from near the center of the Z axis is Occurs in the upward direction.
The magnetic flux 1530b and the magnetic flux 1531b traveling in the same direction are in a repulsive relationship, so when the magnetic flux (1530b, 1531b) reaches the magnetic block 302 with low magnetic resistance, it moves inside the magnetic block 302 with low magnetic resistance. , each proceeding separately in the radial direction.
That is, in FIG. 5B showing the XZ plane, the magnetic flux 1530 is directed to the right of the magnetic block 302 when viewed from the paper, and the magnetic flux 1531 is directed to the left of the magnetic block 302 when viewed from the paper. .

When the magnetic flux reaches the ends (outer periphery, right end, and left end) of the magnetic block 302, correction coils 303 are provided near each end.
As shown in FIG. 5A, the current direction 1501 of the current flowing through the Z coil 301Z and the current direction 1503 of the current flowing through the correction coil 303 are opposite directions.
Therefore, the magnetic fluxes (1530, 1531) of the Z coil 301Z are pushed and bent by the action of the correction coil 303, respectively.
That is, the magnetic flux 1530 is directed downward in the drawing as a magnetic flux 1530a at the right end of the Z coil 301Z (near the correction coil 303).
Further, the magnetic flux 1531 is directed downward in the drawing as a magnetic flux 1531a at the left end of the Z coil 301Z (near the correction coil 303).
As described above, magnetic flux is prevented from reaching the container of the static magnetic field magnet 101, the disk-shaped magnetic pole 201, and the annular magnetic pole 202. Therefore, generation of eddy currents in the container of the static magnetic field magnet 101, the disc-shaped magnetic pole 201, and the annular magnetic pole 202 is suppressed.

Note that when the magnetic block 302 is not provided, the magnetic flux 1530 and the magnetic flux 1531 in FIG. This causes eddy currents to occur.
In addition, when the correction coil 303 is not provided, the magnetic flux 1530a and the magnetic flux 1531a reach the annular magnetic material magnetic pole 202 as shown by the magnetic flux 1522 and the magnetic flux 1523, respectively. This causes eddy currents to occur.
That is, the magnetic flux generated by the Z coil 301Z becomes a factor that prevents desired characteristics from being obtained.

In this way, by keeping the magnetic flux 1530 and the magnetic flux 1531 inside the magnetic block 302 and preventing them from reaching the disc-shaped magnetic pole 201, generation of eddy current in the disc-shaped magnetic pole 201 is prevented, and the MRI image Reduces deterioration of
Further, by preventing the magnetic flux 1530a and the magnetic flux 1531a from reaching the annular magnetic pole 202 by the action of the correction coil 303, generation of eddy current in the annular magnetic pole 202 is prevented, thereby reducing deterioration of the MRI image. .

<Summary of the first embodiment>
According to the open-type magnetic resonance imaging apparatus (open-type MRI apparatus) with this configuration, eddy currents caused by leakage magnetic fields of the gradient magnetic field coils are suppressed without dividing the magnetic poles constituting the static magnetic field magnet, and MRI images image quality can be improved.
Furthermore, since the magnetic poles constituting the static magnetic field magnet are not divided, a predetermined magnetic field strength can be achieved with lower magnetic energy than when the magnetic poles are divided.
Furthermore, since the magnetic poles constituting the static magnetic field magnet are not divided, high processing and assembly precision is not required, and a low-cost open magnetic resonance imaging apparatus can be realized.

<Effects of the first embodiment>
According to the open type magnetic resonance imaging apparatus (open type MRI apparatus) of the first embodiment of the present invention, the eddy current caused by the leakage magnetic field of the gradient magnetic field coil can be generated without dividing the magnetic poles constituting the static magnetic field magnet. can be suppressed and the image quality of MRI images can be improved.
Furthermore, since the magnetic poles constituting the static magnetic field magnet are not divided, a predetermined magnetic field strength can be achieved with lower magnetic energy than when the magnetic poles are divided. That is, the output capacity of the drive power source including the gradient magnetic field coil can be reduced.
Furthermore, since the magnetic poles constituting the static magnetic field magnet are not divided, high processing and assembly precision is not required, and a low-cost open magnetic resonance imaging apparatus can be realized.
Furthermore, since the imaging space 102 of the open type MRI apparatus 100 is not surrounded by structures, the subject 104 can feel a sense of freedom compared to horizontal magnetic field type MRI.
Furthermore, by moving the movable bed 105, the affected part of the subject 104 can be imaged at the center of the imaging space 102 at all times.
Another feature is that an operator of the open MRI apparatus 100 or the like can assist in imaging the subject 104 from outside the open MRI apparatus 100 while an MRI image is being taken.

≪Other embodiments≫
Note that the present invention is not limited to the embodiments described above, and includes various modifications. For example, the embodiments described above are described in detail in order to explain the present invention in an easy-to-understand manner, and the present invention is not necessarily limited to having all the configurations described. Further, it is possible to replace a part of the configuration of one embodiment with a part of the configuration of another embodiment, and furthermore, it is possible to add or add part or all of the configuration of another embodiment to the configuration of one embodiment. It is also possible to delete or replace.
Other embodiments and modifications will be further described below.

《Shield coil》
In FIG. 2, a case has been described in which the shield coil 203 in the static magnetic field magnet 101 is located on the outer diameter side of the disk-shaped magnetic pole 201.
However, the arrangement of the shield coil 203 is not limited to the outer diameter side of the disk-shaped magnetic pole 201. For example, it may be arranged inside the diameter of the disc-shaped magnetic pole 201 and located above the disc-shaped magnetic pole 201. Alternatively, it may be provided in multiple locations.
The shield coil 203 is configured and arranged so that the shield coil 203 acts to reduce the leakage magnetic field outside the open MRI apparatus 100 generated by the magnetic circuit of the disc-shaped magnetic pole 201 and the annular magnetic pole 202. All you have to do is select.

《Generation of static magnetic field》
In FIG. 2, it has been explained that the static magnetic field is generated using a substantially annular main coil 204 made of an electrically conductive material (for example, a superconducting material). However, generation of the static magnetic field is not limited to the method using the annular main coil 204.
For example, a magnetic field corresponding to the magnetic field generated by the annular main coil 204 may be formed using a magnetized permanent magnet (not shown).

《Magnetic block》
In FIG. 3B, a plurality of sheet-like magnetic bodies 302a constituting the magnetic block 302 are shown in the shape of square tiles. However, the shape of the tiles of the sheet-like magnetic material 302a is not limited to a square. For example, it may be configured in a hexagonal shape.
Further, when stacking the sheet-like magnetic bodies 302a layer by layer, the method is not limited to a method in which the upper and lower tiles are stacked in exactly the same shape. The upper and lower tiles may be arranged with their positions shifted.
Further, the shape of the tile of the sheet-like magnetic material 302a may be different between the shape at the center portion of the magnetic material block 302 and the shape at the end portion (circumferential portion).

《MRI equipment other than open type》
With reference to FIGS. 1 and 2, the configuration including the static magnetic field magnet and the gradient magnetic field coil of the first embodiment has been described as being suitable for an open type magnetic resonance imaging apparatus (open type MRI apparatus). However, the configuration including the static magnetic field magnet and the gradient magnetic field coil shown in FIG. 1 can be applied even if it is not an open type.
In other words, the above configuration, which suppresses eddy currents caused by leakage magnetic fields of gradient magnetic field coils, improves the image quality of MRI images, and provides a low-cost MRI apparatus, is applicable not only to open type MRI apparatuses but also to non-open type MRI apparatuses. can also be applied.

100 Open magnetic resonance imaging device, open MRI device 101 Static magnetic field magnet 102 Imaging space 104 Subject 105 Movable bed 106 Support portion 201 Disc-shaped magnetic pole 202 Annular magnetic pole 203 Shield coil 204 Main coil 205 Incline Magnetic field coil 206 RF coil 301 Gradient magnetic field generation coil 301x X coil 301y Y coil 301z Z coil 301xa Left side of X coil 301xb Right side of Coil 402 Magnetic flux generated by the X coil 501 Z coil on the XY plane 502 Magnetic flux generated by the Z coil

Claims (12)

A pair of static magnetic field magnets arranged facing each other centering on the imaging space;
a pair of gradient magnetic field coils arranged opposite to each other with the imaging space at the center;
Equipped with
The static magnetic field magnet is
an annular main coil arranged around the Z axis, which is the direction in which the pair of static magnetic field magnets face each other;
a disc-shaped magnetic pole arranged around the Z-axis ;
an annular magnetic pole arranged around the Z - axis;
Equipped with
The gradient magnetic field coil is arranged closer to the imaging space than the disc-shaped magnetic pole,
The gradient magnetic field coil is
a Z coil that is wound in an annular shape around the Z-axis and provides a magnetic field tilted in the Z-axis direction to the imaging space;
a correction coil wound in an annular shape around the Z-axis outside the Z-coil ;
a magnetic block disposed on the disk-shaped magnetic pole side of the Z coil ;
Equipped with
The magnetic material block is a disc-shaped magnetic material,
The magnetic material block covers surfaces of the Z coil and the correction coil on the disk-shaped magnetic pole side, and the position of the outer periphery substantially coincides with the position of the outer periphery of the correction coil.
An open magnetic resonance imaging device characterized by: In claim 1,
The main coil is arranged outside the disc-shaped magnetic pole and the annular magnetic pole ,
The static magnetic field magnet further includes a shield coil that reduces magnetic field leakage to the outside of the open magnetic resonance imaging apparatus.
An open magnetic resonance imaging device characterized by: In claim 1,
The gradient magnetic field coil is
an X coil that provides a magnetic field tilted in the X-axis direction in the imaging region;
An open magnetic resonance imaging apparatus comprising: a Y coil that applies a magnetic field tilted in the Y-axis direction in an imaging region. In claim 1,
further comprising an RF coil that applies a high-frequency electromagnetic pulse to the static magnetic field in the imaging space;
An open magnetic resonance imaging device characterized by: In claim 1,
comprising a movable bed for transporting a subject to the imaging space of the open magnetic resonance imaging apparatus;
An open magnetic resonance imaging device characterized by: In any one of claims 1 to 3,
The correction coil is arranged radially outside the Z coil,
The correction coil is arranged at the same height as the Z coil in the Z-axis direction.
An open magnetic resonance imaging device characterized by: In claim 1,
The magnetic material block is configured by stacking sheet-like magnetic materials in the Z-axis direction and arranging them in a tile shape in the X-axis and Y-axis directions.
An open magnetic resonance imaging device characterized by: In claim 7,
The sheet-like magnetic material constituting the magnetic material block is a silicon steel plate,
An open magnetic resonance imaging device characterized by: In claim 7,
The magnetic block has a thickness of 20 mm or more,
An open magnetic resonance imaging device characterized by: In claim 2,
The shield coil is arranged outside the diameter of the main coil and generates a magnetic field in the opposite direction to the imaging space.
An open magnetic resonance imaging device characterized by: In claim 3,
The X coil is composed of two coils arranged side by side in the X direction with the Z axis in between,
The Y coil is composed of two coils arranged side by side in the Y direction with the Z axis in between.
An open magnetic resonance imaging device characterized by: In claim 4,
comprising a receiving coil that receives a magnetic resonance signal from the subject generated by a high-frequency electromagnetic pulse applied by the RF coil;
An open magnetic resonance imaging device characterized by:

Images